The wave of the
future for controlling fusion energythe process that powers the
sunmay include more effective harnessing of a fusion device heating
process, using electromagnetic, or radio, waves. An understanding of how
to get "radio-controlled fusion" may come from calculations
done using supercomputers at ORNL.

Controlled fusion
energy, when achieved, could be a favored energy source someday. It would
draw upon an unlimited source of fuelhydrogen isotopes from seawaterand
would produce no greenhouse gases that could adversely affect climate.

The first key to achieving
fusion energy is to heat charged particles (nuclei of hydrogen isotopes)
to very high temperatures such that the electrical repulsion of the nuclei
is overcome during collisions, allowing nuclear fusion reactions to occur.
At these temperatures, the electrons are completely stripped from the
atomic nuclei, yielding an electrically conducting gas called plasma.
The second key is to hold, or confine, the particles and their energy
with magnetic fields long enough for many collisions and reactions to
occur. Such fusion reactions would release enormous amounts of energy
that can be converted to electricity.

In the quest for
controlled fusion, scientists have attained several important milestones.
They have achieved plasma temperatures as high as 520 million degrees,
more than 20 times the temperature at the center of the sun. More than
16 million watts of fusion power have been produced in the laboratory.
The unsolved problem is how to control fusion plasmas to get sustained
fusion reactions. The goal is to prevent the loss of heat from the plasma
center to the edge as a result of irregular fluctuations in plasma velocity
and pressure (turbulence) brought on by the plasma current and other causes.

"Besides heating
the plasma in the way that a microwave oven heats food, experiments show
that radio waves can drive electric currents through the plasma and force
the plasma fluid to flow," says Don Batchelor, head of the Plasma
Theory Group in ORNL's Fusion Energy Division (FED). "These waves
have even been seen to improve the ability of the applied magnetic field
to hold the energetic particles and plasma energy inside the device."

"Radio waves
give us the best 'knob' for precision control of the plasma," says
Mark Carter of FED. "With radio waves we can control where the power
goes, because these waves resonate with the motion of the plasma particles
as they orbit around magnetic field lines. Unfortunately, because the
orbiting plasma particles move at nearly the speed of light, it has been
impossible to calculate how they will respond to radio waves and how much
electric current they will produce."

To address this
problem, Fred Jaeger and Lee Berry of FED, working with Ed D'Azevedo of
ORNL's Computer Science and Mathematics Division, developed a computer
program for the IBM supercomputer of the Department of Energy's Center
for Computational Sciences at ORNL to compute plasma waves across the
entire cross section of a fusion plasma. The program solves an enormous
set of equations, providing the first two-dimensional (2D), high-definition
picture of radio waves injected from an antenna into the plasma of a doughnut-shaped
tokamak. Using 576 processors at speeds of up to 650 billion operations
per second, the program shows that, at certain locations, the waves shift
from a long-wavelength to a short-wavelength structure (mode conversion)
and become rapidly absorbed by the plasma. The group has recently created
a 3D code for this modeling that could lead to a method of fine tuning
the injection of the waves to maximize control of the plasma.

Above:
Top view of the Quasi-Poloidal Stellarator (QPS), which is an optimized,
low-aspect-ratio fusion stellarator device. The plasma shape (colors
indicate magnetic field strength) and filamentary magnetic coils
(light blue) are shown. Below: Side view of the QPS optimized low-aspect-ratio
stellarator device. Visualizations by Don Spong.

Another area in
which FED scientists are using supercomputers to advance fusion research
is in the analysis of very complex, nonsymmetric magnetic systems for
plasma containment, called stellarators. These are shaped like a cruller
wrapped with twisting magnetic coils. The ORNL supercomputers were used
in the analysis and design of a new type of magnetic fusion device called
the Quasi-Poloidal Stellarator (QPS). QPS will use a much smaller plasma
current and rely more heavily on external coils to provide the needed
magnetic fields for plasma confinement. This device may result in a much
smaller and more economically attractive fusion reactor than existing
stellarators and would eliminate the potentially damaging plasma disruptions
that plague conventional research tokamaks. It is hoped that QPS will
be built at ORNL, using DOE funds, starting in 2003.

"We are employing
a Levenberg-Marquardt algorithm on the IBM supercomputer to calculate
how to modify the plasma shape to optimize energy transport and plasma
stability," says Don Spong of FED. "Once we have determined
the best shape, then we will infer the design of external magnetic coils
that can be engineered cost effectively to achieve that shape."

"We need supercomputers
to model as many as 40 variables that interact with each other to describe
the plasma," Batchelor says. "We are twiddling 40 knobs at the
same time computationally to get six or more competing physical properties
simultaneously as good as they can be."

The beauty of the
new technique developed to study waves is that it can be extended to 3D
plasmas such as those in stellarators. These are significantly more complicated
in shape than the tokamak, the present state of the art for plasma wave
computations.

With the help of
ORNL's supercomputers and new funding from DOE's Scientific Discovery
through Advanced Computation (SciDAC) initiative, fusion researchers at
ORNL are likely to make waves in this important energy research field.